1. Field of Invention
The present invention relates to a method of preventing strength reduction due to phase conversion in high alumina cement products, and more particularly, to a method of preventing hydrogarnet (C.sub.3 AH.sub.6) formation and promoting stratlingite (C.sub.2 ASH.sub.8) formation in high alumina cement products by adding a novel conversion-preventing additive to high alumina cement compositions; the invention further relates to the novel conversion-preventing additives, to blended high alumina cement compositions (products) containing said conversion-preventing additive, and to a method for pre-treatment of natural zeolite for use in the conversion-preventing additives.
2. Description of Prior Art
High alumina cement (HAC) was invented in 1908 by J. Bied, a director of the Pavin de Lafarge company, France (see French patents 320290 and 391454 to J. Bied). Spackman was the first to obtain a U.S. patent for this material (see, Proc. Amer. Soc. Test. Mater. 10, 315, 1910, H. S. Spackman). Commercial HAC was produced by the Lafarge company in 1913. This cement is based upon calcium monoaluminate (CA). Hydration of CA eventually results in the formation of cubic hydrogarnet (C.sub.3 AH.sub.6) crystalline phase, a thermodynamically stable aluminate hydrate. High early strength, good chemical resistance and high temperature resistance of HAC products has encouraged the use of high alumina cement concrete in certain construction engineering applications. However, conversion of hexagonal calcium aluminate hydrates (CAH.sub.10 or C.sub.2 AH.sub.8) to cubic hydrogarnet (C.sub.3 AH.sub.6) in hydrated HAC concrete under certain temperature conditions has been a major problem limiting its use. The conversion process results in a densification of over 50%. Consequently, a significant reduction of strength during the service life of the concrete occurs. Many high alumina cement concrete structures built in the UK in the 1940's and 1950's collapsed due to later conversion. As a direct result of those investigations, the British government issued a document on Jul. 20, 1974, Ref. BRA. 1068/2, entitled "High Alumina Cement Concrete in Buildings". This document stated that "high alumina cement should not be used for structural work in buildings until further notice". The application of high alumina cement in structural members has been banned in virtually every country in the world.
The conversion process in HAC systems has been extensively studied for decades. A HAC-based blended cement containing about equal amount by weight of granulated blast-furnace slag (GBS) was commercialized by the British Research Establishment (BRE) under the trademark "BRECEM". It was touted as a possible solution for prevention of the conversion reaction (Majumdar, A. J., et al., Cem. Concr. Res., 19, p.779, 1989; 20, p.197, 1990; 22, p.1101, 1992; 24, p.335, 1994). This effect was attributed to stratlingite formation in preference to the hydrogarnet. Other pozzolanic additives such as silica fume and fly ash have also been reported to favour the formation of stratlingite.
The early strength of such material is significantly reduced as a great amount of HAC is replaced by granulated blast-furnace slag. The strength slowly increases to a value approaching the one-day strength of HAC concrete at 1-2 years when water-cured at 20.degree. C. Hydrogarnet was still formed at 38.degree. C. although the HAC-slag blended cement concrete had no strength reduction. Lafarge Calcium Aluminates, Virginia, USA, concluded that "The approach to aluminous cement concrete that seeks to avoid conversion is unrealistic" (See, "Calcium Aluminate Cements", Chapman and Hall, New York, p. 206, C. M. George, 1990).
A new cement resulting in direct formation of stable hydrogarnet and stratlingite phase by fusing limestone, alumina, and silica sand and quenching the resulting glass was described in U.S. Pat. No. 4,605,443 issued 1986 to J. F. MacDowell. This direct hydration avoids the formation of metastable hydrates that gradually react to weaken the calcium aluminate cement system during conversion. According to that patent, a predominantly hydrogarnet paste had compressive strength between 35 and 70 MPa, and 20-35% porosity. The stratlingite-rich paste had strength between 70 and 140 MPa and 1-10% porosity. Also according to that patent, the cement had fast hardening characteristics and good thermal and chemical stability similar to high alumina cement. High strength developed during the first 24 hours and then remained constant or decreased slightly. The application of this technology was reported to be delayed. The delay was attributed to the high melting temperatures required and the lack of knowledge of relevant engineering properties.